The present invention relates to an improvement in the process for production of carboxylic acids, esters and anhydrides by carbonylation in the presence of catalyst complex systems comprising a rhodium-containing or an iridium-containing component and a halogen component. More particularly, it relates to a method for regenerating the metal catalyst complex employed in such processes.
A process has recently been developed for the preparation of carboxylic acids, esters and anhydrides by the reaction of olefins, alcohols or esters, or ethers and halide derivatives thereof, with carbon monoxide in contact with catalyst systems formed on mixing of a rhodium component or an iridium component and a halogen component in the presence of carbon monoxide. This process is described, for example, in U.S. Pat. Nos. 3,579,551, 3,579,552, 3,769,329 and 3,772,380. A hydrogen halide or an alkyl halide, notably hydrogen iodide or methyl iodide, is usually employed as the halogen component in these catalyst systems. The catalyst itself is self-regenerative. In operation of the process on a continuous basis, a solution containing the soluble catalyst complex is separated from the reactor effluent and recycled to the reactor. However, with operation over extended periods of time, corrosion products, namely, iodides of iron, nickel, molybdenum, chromium and the like form and build up in this catalyst recycle stream. Such foreign metals, if present in sufficient quantity, are known to interfere with the carbonylation reaction or accelerate competing reactions such as the water-gas reaction (carbon dioxide and hydrogen formation) and alkane formation. These have an adverse effect on the process in that they cause a decrease in the partial pressure of carbon monoxide in the reactor resulting in loss in yield based on carbon monoxide. Further, foreign metals present can react with ionic iodine thus making this component of the catalytic system unavailable for reaction with rhodium or iridium, causing instability in the catalyst system. In view of the high cost of the rhodium- or iridium-containing catalyst complex, replacement of spent catalyst complex can be effected only at a prohibitive cost. Consequently, a method for regeneration of the catalyst complex is not only desirable but necessary. It is an object of the present invention, therefore, to provide a method for treating spent catalyst solution to remove metallic corrosion products therefrom and to recover the valuable rhodium metal or iridium metal component in a form suitable for return to the process as active catalyst complex.